Cell Cycle and Nucleus Overview

Study Notes

The nucleus contains the blueprint for all cell structures and activities, encoded in the DNA of chromosomes. It also contains the molecular machinery for DNA replication and the synthesis and processing of RNA (rRNA, mRNA, tRNA).
The nucleus itself does not produce proteins; proteins needed for nuclear activities are imported from the cytoplasm.
The nucleus usually appears as a rounded or elongated structure in the cell center.
The nucleus's main components are the nuclear envelope, chromatin, nucleolus, and nuclear matrix.
The nucleus appears intensely stained dark blue or black in common hematoxylin and eosin-stained preparations.

The nucleus is surrounded by two parallel membranes, separated by a space called the perinuclear cisterna.
The paired membranes and the intervening space form the nuclear envelope.
The nuclear envelope is continuous with the endoplasmic reticulum, shown by polyribosomes attached to the outer membrane.
Nuclear pores exist where the inner and outer membranes of the envelope fuse, allowing substances to pass between the nucleus and cytoplasm.
Only ions and small molecules pass freely through nuclear pores; larger molecules require an active transport process.

Chromatin is composed mainly of DNA coiled around basic proteins called histones.
The basic structural unit of chromatin is the nucleosome.
Four types of histones form the core of a nucleosome, with DNA base pairs wrapped around them.
Linker DNA connects adjacent nucleosomes, and another histone type is bound to this DNA.
Chromatin can exist in two readily distinguishable forms: euchromatin (genetically active) and heterochromatin (genetically inactive).
Heterochromatin appears electron-dense and basophilic.
Euchromatin appears electron-lucent and lightly stained.
The proportion of heterochromatin to euchromatin affects the nucleus's overall intensity of staining, which can be used to interpret its functional state.

The nucleolus is a spherical structure rich in rRNA and proteins, appearing basophilic.
It is the site where ribosomal subunits are synthesized.
Ribosomal proteins are synthesized in the cytoplasm and join with rRNAs in the nucleolus.
Ribosomal subunits then migrate to the cytoplasm through nuclear pores.

The cell cycle is a series of events preparing a cell to divide into two daughter cells.
Interphase is the long period of growth and DNA replication before cell division.
Interphase consists of three stages: G1, S, and G2.
- G1: Cell growth, synthesis of macromolecules essential for DNA duplication.
- S: DNA is duplicated.
- G2: Cell undergoes preparations for mitosis.
Mitosis is the shorter period where the cell's nucleus divides, leading to two daughter cells.
Cells that exit the division cycle enter a resting stage, called G0.